Method of manufacturing chromium alloyed steel

ABSTRACT

A CHROMIUM ALLOYED STEEL IS MANUFACTURED IN THIS WAY. MOLTEN IRON, CONTAINING CARBON AND ALLOYING SUBSTNCES, IS MIXED WITH CHROMIUM OXIDE AND A SLAG FORMING AGENT, THE ALLOYING SUBSTANCES ARE OXIDIZED TO FORM SLAG. THE CHROMIUM OXIDE IS REDUCED TO METALLIC CHRONIUM. THE SLAG IS REMOVED. THE MOLTEN METAL IS NOW REFINED BY MEANS OF OXYGEN, RESULTING IN THE CARBON CONTENT DROPPING FROM 2 TO 0.2% AND THE TEMPERATURE INCREASING TO 1700*C.

United States Patent METHOD OF MANUFACTURING CHROMIUM ALLOYED STEEL Erik Anders Ake Josefsson, Borlange, Sweden, assignor to Stora Kopparbergs Bergslags Aktiebolag, Falun, Sweden No Drawing. Filed Feb. 17, 1970, Ser. No. 12,143 Claims priority, applicatiogl/ggveden, Apr. 23, 1969,

Int. Cl. czzc 33/00, 39/14 US. Cl. 75-1305 11 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a method of manufacturing chromium alloyed steel. Chromium alloyed steel is at present normally manufactured by melting steel scrap circulating in the factory together with alloyed or unalloyed merchant scrap and ferro alloys in an arc furnace, after which the melt is metallurgically processed to obtain the desired type of steel. The price of ferro alloys increases as the percentage of impurities decreases. Since the cheaper types of ferro chromium alloys contain considerable quantities of carbon whereas for most chromium steels there must be an extremely low carbon content, it is a considerable economic advantage if it is possible to reduce considerable quantities of carbon during the manufacture of chromium steel. This can be done with oxygen gas at high temperatures in a process which has been used for a long time in electric furnaces. Since, however, electric steel furnaces are not very suitable as refining furnaces it has also been suggested to manufacture chromium alloyed steel, for example stainless steel, in stationary oxygen gas converters intended for the so-called LD process. In this case liquid alloyed or unalloyed pig iron is used as starting material, which is converted to chromium alloyed steel by refining with oxygen gas and the addition of ferro alloys, and/0r scrap.

Gaseous oxygen is used for this purpose in order to oxidize not only the carbon but also the silicon content of the charge. This high-quality reducing agent is thus only used to generate heat. The silica formed is absorbed in a calcium silicate slag which, even if it is tapped at the start of the blowing, causes considerable metal losses since with conventional oxygen refining iron and chromium are oxidized to a considerable extent at the same time as the silicon. On the other hand, if the slag is allowed to remain until completion of the refining, the silica formed in the initial stage causes a considerable increase in the quantity of slag, particularly if a high CaOzSiO ratio is desired in the final stage. The large quantity of slag causes several other disadvantages; for instance it takes up great quantities of chromium oxidized during the carbon refining which must then be reduced to chromium again after completed refining. For this reason ferro chromium alloys poor in silicon are preferred as the chromium raw material, but these are more expensive to manufacture than those rich in silicon.

In the present invention the silicon in the ferro chromium is used, possibly in combination with silicon in the melt or added to the melt, or with other substances having greater afiinity to oxygen than chromium, in order to re- 3,711,278 Patented Jan. 16, 1973 "ice duce chromium from chromium ore and thus increase the chromium content of the melt. With suitable equipment and technique this process can be carried out so that the slag formed has low percentages of chromium and iron and can thus be separated with low metal losses. Because the silica formed is removed before the carbon refining, a considerably smaller quantity of slag is formed during the refining than with other methods. The invention thus makes it possible, without any drawbacks, to use cheap metallic chromium raw materials such as ferro chromium alloys having high percentages of silicon and carbon which are cheaper to manufacture, and also makes it possible to utilize the silicon content of these products to reduce chromium from chromium ore, thus further decreasing the costs of the raw materials for thesteel.

According to the invention a carbonaceous iron metal is used as starting material which contains alloying substances having a greater afiinity to oxy gen than chromium. In a first stage this melt is brought by intensive stirring into intimate contact with a material containing chromium oxide which is added in such quantities that substantially the entire mass of said alloying substances is oxidized and turned to slag with added slag former, while substantially all the chromium oxide added is reduced to metal and is consequently dissolved in the iron melt. The melt is deslagged and in a second stage it is refined with oxygen or oxygen-enriched air, the carbon content of the metal at the start of the refining being at least 2% and the temperature of the melt during the refining being permitted to increase as the carbon content drops so that the temperature is at least 1700 when the carbon content has fallen to 0.20%. Thanks to the introduction of the first stage, the refining process in the second stage can be carried out under the conditions stipulated with considerably less slag formation thus decreasing the chromium oxidation, and thus also the necessity for re-reduction of oxidized chromium after completed refining.

As mentioned, the original iron melt should contain alloying substances having a greater aflinity to oxygen than chromium, and one such suitable substance is silicon. The starting melt may suitably consist of a siliceous pig iron. A favourable method of producing siliceous pig iron is to melt scrap in a cupola furnace with acid lining. This pig iron may possibly be desulphurized. Alloying substances having greater alfinity to oxygen than chromium may also be added, suitably partly in the form of iron alloys with high percentages of carbon and silicon, such as charge chrome.

Under certain circumstances it may also be economical to add ferro-silicon.

The material containing chromium oxide, which is added in the first stage according to the invention, suitably consists of chromium ore. According to the invention it is advisable to adjust the quantity of alloying substances having greater affinity to oxygen than chromium so that they are substantially consumed for the reduction of chromium oxide and other similar or more easily reduced oxides in the material containing chromium oxide and so that in this way a chromiferrous melt is obtained after the first stage which has such a chromium content that, possibly after further additions of chromium during the second stage, the desired final percentage of chromium is obtained in the material. It is then unnecessary to make any great adjustment of the chromium content in the melt after the second stage. Amongst the easily reduced oxides which may exist in the material containing chromium oxide may be mentioned particularly iron and nickel oxides.

The temperature during the first stage should suitably be above 1450 C. and may possibly be adjusted by the combustion of hydrocarbon, preferably with oxygen, above the surface of the melt. By preheating and possibly melting the additives it is possible to further increase the quantity of chromium which can be reduced from the ore.

In the first stage basic slag formers are suitably added, preferably containing CaO and MgO,to produce a basic, easily flowing slag which is removed after the first stage.

Before the second stage more carbon may be added to the melt to generate heat if necessary. On the other hand, in order to prevent the temperature increasing too rapidly during the second stage, coolant is suitably added, preferably material containing chromium or chromium oxide, thus further increasing the chromium content in the melt. The ratio between the temperature of the melt and its carbon content which should be aimed at in order to give the least possible chromium oxidation can be obtained from the theoretically determined conditions of equilibrium in such melts. The temperature in the Charge should not increase more rapidly of higher than necessary as this causes increased wear on the lining. Before and during the second stage slag former is added so that a slag with a basic value greater than 3.5 is obtained. By basic value is meant in this specification the ratio (Ca+MgO) :SiO these substances being indicated in percent by weight. Since the alloying substances having a greater affinity to oxygen than chromium are removed in the first stage and,

since the temperature is kept so high that the chromium oxidation is as little as possible, the econd step can be carried out with very little slag, thus facilitating the carbon refining.

The second stage is carried out entirely or partly during strong mechanical stirring.

Chromium which may have been oxidized is suitably recovered during the second stage by reducing the slag by the addition of reducing agent having a greater aflinity to oxygen than chromium, for example ferro-silicon or some other silicon alloy having low carbon content. It

should be ascertained that the basic value of the slag after completed reduction is higher than 2.

The reduction after the second stage suitably takes placeduring strong mechanical stirring.

The intensive mechanical stirring which should be used in the first stage and partly or entirely during the second stage and also during the reduction after the second stage can be elfected by using a rotary furnace whose axis of rotation slopes at an angle of, at the most 45, to the horizontal plane, and preferably rotating the furnace rapidly, that is at more than rpm. The speed may, of course, not be so high that the stirring effect is lost due to the influence of centrifugal forces on the melt. When such a rotary furnace is used, part of the refining process takes place when the furnace is stationary and standing in vertical position and oxygen gas is blown from above towards the surface, thus making it possible to effect a so-called burning spot where the carbon isrefined at extremely high temperatures.

The intensive mechanical stirring of the metal can also be effected by producing circular movements in the melt in a substantially cylindrical container having a substantially vertical axis. One example of such a means is the so-called shaking ladle. Inthis case also, part of the refining process may be carried out While-the container is stationary.

The use of the invention will be illustrated here by a number of examples.

EXAMPLE 1 A KALDO-furnace was charged with liquid pig iron having the following composition:

Percent C 4.00 Si 1 .00 Mn 0. 80 P 0.20

Fe Remainder About 25 kg. each of iron-ore concentrate and quicklime was added per ton of pig iron and oxygen gas blowing was initiated during rotation of the furnace in order to remove phosphorus from the melt so that a maximum of 0.025% remains, while retaining the highest possible C content.

During the blowing 130 kg. nickel oxide were added per ton of pig iron, which forms a part of the total requirement of nickel for the manufacture of stainless steel of 18/ 8 type. Because of the addition of NiO at this stage it is possible to maintain relatively low temperatures in the melt, thus contributing to the dephosphorization without too much of the carbon being oxidized.

When about 55 m n oxygen gas per ton of pig iron had been blown over the melt in the rotating furnace the melt had obtained the following approximate analysis:

Percent Si 0 Mn 0 1 Fe Remainder The temperature of the melt was approximately 1550 C. The final temperature can be kept lower if desired by additional cooling with NiO Within the limits for the stipulated final Ni content of the steel or by means of ordinary steel scrap, preferably of known composition.

The phosphorous slag was then tapped off. It should be pointed out in this connection that it is possible to dephosphorize the melt in two or more stages, tapping oif the slag between each stage.

The melt pre-treated in this way was then used to manufacture a chromium alloyed steel according to the invention. In a first stage the furnace was rotated rapidly while 325 kg. charge chrome (6% C, 8% Si, 54% Cr, 32% Fe),

kg. chromium ore (30% Cr, 19% Fe) and '70 kg. calcium were added successively. In order to maintain the temperature of the melt, heat was supplied by means of an oxy-fuel gas burner. After 10 minutes the silicon had reacted with the chromium oxide in the chromium ore. The melt obtained, which had a temperature of 1500 C., contained:

Percent content dropped to 0.15%. After completed blowing the chromium content in the melt was 16-5 While approximately 60% of the chromium added in stage 2 remained in the slag, which had a basic value of 4.3.

The melt was cooled to a temperature of around 1600- C. by the addition of approximately 300 kg. stainless scrap having a similar composition to the finished steel,

While the furnace was rotated. In orderto reduce the chromium of the slag again, about 20 kg. 50% FeSi were added per ton of melt. The reduction was carried out during rotation of the furnace. The heat developed was sufiicient to melt the FeSi added, substantially maintaining the temperature of the melt. The finished steel contained about 18% chromium and about 8% Ni. It was tapped,

during deoxidation by the addition of silicon calcium.

EXAMPLE 2 A crude iron having the following composition was manufacturfed in a blast-furnace from chromium ore, iron ore and raw material, for instance nickel oxide, con- The crude iron was charged at a temperature of about 1500 C. in liquid form into an oxygen gas converter of LD/KALDO type in which the oxygen gas refining can take place with the converter either in stationary, vertical position or rotating with a sloping axis.

The melt of liquid crude iron was processed according to stage 1 of the invention by allowing the furnace to rotate and successively adding 180 kg. chromium ore and 70 kg. quicklime. The chromium was thus reduced from the ore by the Si in the crude iron. After minutes an iron melt with the following composition had been obtained:

The slag had a basic value of 1.24 and contained 3.7% Cr O When the slag had been tapped off the converter was placed in vertical position and oxygen gas blowing was initiated while about 200 kg. stainless scrap of 18/ 8 type and 30 kg. quick-lime were successively added per ton of charged crude iron. Because of the low silicon content in the charge very little slag was formed and the re fining took place extremely rapidly, forming an extremely hot burning spot, thus reducing the oxidation of chromium. When about 75 m n oxygen gas per ton crude iron had been blown onto the melt, it had approximately the following composition:

Percent C, ca 0.05 Si 0 Cr, ca 17 Ni, ca 7.7 Fe remainder The temperature of the melt had now risen to about 1800. Some of the added chromium had been oxidized and remained in the slag. The chromium in the slag was then reduced again by the addition of about 35 kg. 50% FeSi and 45 kg. quicklime per ton of crude iron white the converter was rotated in sloping position. Thanks to the stirring of the melt the chromium content in the slag is thus reduced to below 3%. At the same time the melt was cooled to a temperature of about 1600 C. by the addition of approximately 230 kg. stainless scrap of 18/ 8 type per ton of crude iron and rotation of the furnace. The finished steel contained about 18% Cr and 8% Ni and was tapped after deoxidation with silicon calcium.

EXAMPLE3 In a suitable melting furnace, for example a cupola furnace with acid lining or an arc furnace 750 kg, hematite pig iron containing 3.5% C and 1.5% Si was melted together with 230 kg. unrefined chromium containing 6% C, 8% Si and 54% Cr. The pig iron may be replaced by a corresponding quantity of scrap and carburizer, for example coke. The melt was tapped and, possibly after desulphurizing, charged into a shaking ladle at a temperature of 1450 C. In the ladle a mixture of 70 kg. quicklime and 180 kg. chromium ore (44% Cr O 24.5% FeO) was added. The ladle was made to shake so that the reaction between lime and ore on the one hand and the silicon in the melt on the other hand was completed and a calcium silicate slag poor in chromium and iron was formed. When the reaction was completed, which took about 15 minutes, the metal melt contained 3.9% C, 0.2% Si, 16.8% Cr, and a slag could be tapped off containing 3.2% Cr -O and 2% fe and having a basic value of 1.35.

When the slag had been tapped oif the melt was refined during stirring by blowing oxygen gas onto the surface. 15 kg. ferro chrome carbure with low Si content and kg. quicklime were added successively. After further refining with oxygen gas the temperature of the melt Was 1800 and its composition was as follows:

Percent C 0.05 Si 0 Cr 16.8 Fe remainder The slag was reduced by the addition of 2 0 kg. 50% FeSi. The melt was cooled to the desired tapping temperature by the addition of stainless scrap. Tapping was done through a tap-hole so that the slag could be kept in the ladle and be used as the material containing chromium oxide in stage 1 for the next charge.

What is claimed is:

1. A method for manufacturing chromium alloyed steel comprising: admixing intimately, during a first chromium oxide reducing stage, while intensively stirring a carbonaceous iron melt with silicon as an alloying substance having a greater affinity to oxygen than chromium said silicon being furnished to the melt from compositions selected from the group consisting of silicon, ferrosilicon, and charge chrome and also admixing said melt with a charge comprising chromium oxide in a quantity whereby substantially the entire silicon in said alloying substance is oxidized and converted, by an added slag former of CaO or MgO, into slag, and whereby substantially all the chromium oxide in the material containing chromium oxide is reduced to metal; adjusting the quantity of silicon in the iron melt whereby the chromium reduced by silicon, together with any chromium already in the iron melt and chromium added during the continued treatment of the melt produce adesired final percentage of chromium in the material; maintaining during said first oxidizing stage a temperature of more than 1450 C. in said iron melt; deslagging said iron melt; refining said iron melt in a second stage with oxygen or oxygen enriched air, whereby the carbon content in said melt at the start of the refining is at least 2% and the temperature of the melt is being allowed to increase as the carbon content of said melt drops during refining whereby the temperature of the melt is at least 1700 C. when the carbon content of said melt has fallen to 0.20%.

2. The method defined in claim 1 and wherein the carbonaceous iron melt consists of a siliceous pig iron.

3. The method defined in claim 1 and wherein chromium ore is used as the material containing chromium oxide.

4. The method defined in claim 1 and wherein the temperature during the first stage is maintained by the combustion of hydrocarbon with oxygen.

5. The method defined in claim 1 and wherein said basic slag forms of C210 and MgO produces an easily flowing slag with a basic value of 0.8 to 1.8 during said first stage.

6. The method defined in claim 1 and wherein carbon is added to the melt before the start of the second stage.

7. The method defined in claim 1 and wherein refining during the second stage takes place partly or entirely with strong mechanical stirring.

8. The method defined in claim 1 and wherein reduction of the slag from the second stage is carried out by means of strong mechanical stirring.

9. The method defined in claim 1 and wherein the mechanical stirring of the melt is efiected in a. rotary furnace whose axis of rotation slopes at the most 45 to the horizontal plane and the furnace is preferably made to rotate rapidly at more than 10 r.p.m.

10. The method defined in claim 9 and wherein at least part of the second stage, refining process is carried out with the furnace stationary in vertical position;

11. The method defined in claim 1 and wherein mechanical stirring of the melt is effected by producing circular movements in the melt in a substantially cylindrical container having a substantially vertical axis.

References Cited UNITED STATES PATENTS 1,354,491 16/1920 Johnson 75-l30.5 1,444,062 2/1923 Carney 75l30.5 2,238,078 4/1941 Royster 75-l30.5 X 2,286,577 6/1942 Royster 75130.5 X 2,847,301 8/1958 Shaw 75-130.5 3,198,624 8/1965 Bell 75-1305 X 3,366,474 1/1968 lAkita 75--l30,5

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner U.S. c1. X.R. 75 52, 6O 

